The γ -ray Emissivity of the Local Interstellar Medium from Correlations with Gas at Intermediate Latitudes

Abstract

A survey of recent studies of the correlation between γ -rays from latitudes |b| > 10 degrees and gas tracers is presented. Results for the ranges 35-100 MeV and above 100 MeV from the SAS-2 satellite, and for energies between 70 and 5000 MeV from the COS-B satellite, are used to obtain an estimate of the γ -ray emissivity spectrum for all forms of gas. Good agreement between the two experiments is found. A comparison is made between this spectrum (which is an average for a region some few hundred parsecs around the Sun) and that expected for recent estimates of the low energy electron spectrum in the local interstellar medium. If the pion-decay component is as expected for the demodulated interplanetary proton spectrum, then the electron spectrum must have a steep slope (differential index 2.8) below 1 GeV. If the pion contribution is smaller than expected, however, a flatter electron spectrum is allowable. The presence of a component of γ -ray emission related to gas in molecular form is evident in both the SAS-2 and COS-B data. We discuss the correlation of the SAS-2 data with both components and show that the emissivities of each component can be independently determined. The longitude dependence of the emission is also discussed. Finally, an examination of the γ -ray fluxes from specific dense clouds of molecular gas is made.